Many welding applications such as MIG (metal inert gas) or GMAW (gas metal arc welding) utilize a wire feeder to provide filler metal to the weld. Generally, the wire feeder will provide wire at a nominally constant speed (typically given in inches per minute). Wire feed speed controllers control the speed at which the wire is fed to the arc.
A typical prior art wire feeder includes a motor that pulls wire from a reel and feeds the wire at a wire feed speed to the weld arc. The motor is controlled by a wire feed controller that may be a stand alone controller or may be part of a controller that controls other aspects of the welding process. The wire feed controller controls the speed of the wire feeder and typically includes a potentiometer (or digital up/down input buttons) on a front panel of the controller which the user uses to set wire feed speed.
A user selectable input, such as the angular position of a knob, typically determines the resistance of the potentiometer, which is used to set the speed point in the control circuit. Digital systems typically provide the output of an up/down button or other input device to a microprocessor or digital control device. The controller may include feedback circuitry to control the wire feed speed, or the speed control may be open loop.
Generally, the wire feeding system has a response of the wire feed speed relative to the user selectable input. For example, as the user turns the front panel potentiometer a given angular rotation the wire speed the changes a given amount. The response is dependent upon the type of control and the components used to implement the control.
The response of the wire feed speed relative to the potentiometer setting may be described as having a sensitivity: inches/minute/degree of angular rotation of the potentiometer (or user selectable input), which is the relationship between angular position and wire feed speed. The sensitivity is also the slope of the potentiometer versus wire feed speed curve, for a given potentiometer setting.
Additionally, a response may be described as having a slope over a range, which is the average slope of the potentiometer versus wire feed speed curve over that range. When the curve is linear over the range, the response of the wire feed speed relative to the user selected input is said to be linear. Conversely, when the curve is not linear over the range, the response of the wire feed speed relative to the user selected input is said to be nonlinear.
Given the wide variety of welding applications, processes and power supplies, a wide variety of sensitivities is desirable. Some prior art wire feed controllers created two sensitivities by providing a toggle switch to select between a faster range and a slower range. Thus, the angular sensitivity at slower speeds is greater than when using the potentiometer for the full range. However, this requires an additional control switch. Also, this prevented the use of a direct wire feed speed reading, since a single potentiometer knob was used for multiple wire feed speed ranges.
Another prior art system that had multiple sensitivities is described in U.S. patent application Serial No. 08/911,998, now U.S. Pat. No. 5,990,447, which is owned by assignee of this invention, and which is implemented in the Miller.RTM. Millermatice.RTM. 300 welding power source, has a controller for the wire feed speed that is inherently linear. A nonlinear circuit input circuit provides a variety of gains depending upon the setting of the potentiometer, to create a nonlinear response of the wire feed speed relative to the potentiometer setting, because it is desirable, for the applications for which that welding power source is often used, to have the sensitivity of the potentiometer be greater at lower speeds than at higher speeds.
Conversely, some welding applications and processes may be better implemented when the sensitivity of the user selectable input is constant over an entire range. The Hobart Handler.RTM. 120/150, for example, is often operated at the lower end of its wire feed speed range. A nonlinear response, with less sensitivity at the slower speeds, makes it difficult for the operator to achieve the desired wire feed speed (WFS) settings. Thus, a linear response is desired. However, not all controllers provide a linear relationship between the input in output. Thus, it may be desirable to provide a nonlinear stage which corrects for inherent nonlinearity in a controller and/or motor, and create a desired overall response. The combined effect of the nonlinear stage and the inherent nonlinearity may be a substantially linear controller, or one with a desired nonlinearity.
It is typical in the prior art to control a motor i.e. using a pulse width modulated integrated circuit. While such circuits may be designed to be inexpensive, when they are used to dictate a desired response of the motor relative to the user input they are often expensive and require external circuitry to condition the signal coming from a potentiometer. Given the number of components, such a system has increased risk of failure and may be expensive. Thus, it would be desirable to have the response of the motor to the user input be determined by the input circuit.
Given the variety of needs for linear or nonlinear responses, a controller having a user selectable input which may be tailored to a particular response is desirable. Preferably, such a user selectable input will be relatively inexpensive to implement, and not be complicated and require an excessive number of components.